1. Field of the Invention
The present invention relates to a method for testing the surface status of a sample (a semiconductor device, etc.) and apparatus therefor and particularly to an inspection method and apparatus therefor for imaging and inspecting fine pattern defects on the surface of a semiconductor device in high sensitivity and high resolution at high speed using an electron beam.
2. Description of the Prior Art
As an inspecting method for detecting defects of a circuit pattern formed on a wafer by comparison test in the manufacturing process of a semiconductor device, there is a method for obtaining images of two or more same kind of LSI patterns on one wafer using light, comparing these plurality of images, and testing existence of pattern defects and it is already put into practical use. The outline of this inspecting method is described in “Monthly Semiconductor World”, October issue, 1995, pp. 114 to 117. When pattern defects in the manufacturing process of a semiconductor device are tested by such an optical inspecting method, residuals of a silicon oxide film through which light transmits and a photosensitive resist material cannot be detected. Residual etching below the resolution of the optical system and a nonopening defect of a fine conducting hole can be neither detected.
To solve such a problem in the optical inspecting method, a pattern comparison inspecting method using an electron beam is described in Japanese Patent Application Laid-Open 59-192943, J. Vac. Sci. Tech. B, Vol. 9, No. 6, pp. 3005–3009 (1991), J. Vac. Sci. Tech. B, Vol. 10, No. 6, pp. 2804–2808 (1992), SPIE, Vol. 2439, pp. 174 0 183, and Japanese Patent Application Laid-Open 05-158703. In this case, to obtain a practical inspecting speed, it is necessary to obtain pattern images at a very high speed. To reserve the S/N ratio of images obtained at high speed, a beam current more than 100 times (more than 10 nA) of that of a normal scanning electron microscope is used.
In the aforementioned prior testing art using an electron beam, to form images maintaining the S/N ratio which can be tested, an electron beam having a large current is used. However, since the electron beam is limited to a spot shape and this spot beam is two-dimensionally scanned on the surface of a sample, there is a limit to the high speed (shortening of the inspecting time). There is also a limit to the large current of an electron beam to be used due to the brightness of the electron source used and space charge effect. For example, to obtain a resolution of about 0.1 μm, the electron beam current to be used is theoretically limited to about several hundreds nA and only about 100 nA can be actually used. The S/N ratio of an image is decided by the number of electrons to be used so as to form the image, that is, the product of beam current and time required to obtain the image. In consideration of necessity of reservation of the S/N ratio on the image processing ready level, to obtain a resolution of 0.1 μm at a beam current of 100 nA, about 100 seconds or more are required to test an area of 1 cm2 of the surface of a sample. On the other hand, in the aforementioned conventional optical inspection apparatus, the test required time for an inspection area of 1 cm2 is very short such as about 5 seconds.